1. Field of the Invention
The present invention relates to a biaxially-optical polynorbornene-based film and a method of manufacturing the same, an integrated optical compensation polarizer having the film and a method of manufacturing the polarizer, and a liquid crystal display panel containing the film and/or the polarizer. More particularly, the present invention relates to a biaxial-optical polynorbornene-based film having high light transmittance, a uniform in-plane retardation value, and a negative retardation value in a thickness direction, thereby acting both as a negative C-plate and as an A-plate to facilitate a reduction in the thickness of liquid crystal display panels and a simplified manufacturing process, and a method of manufacturing the film, an integrated optical compensation polarizer having the film and a method of manufacturing the polarizer, and a liquid crystal display panel containing the film and/or the polarizer.
2. Description of the Related Art
Liquid crystal display panels are operable for several hours even by battery due to their low power consumption, require a small space due to their small volume, and are easily portable due to their light weight, enabling them to be widely used in televisions, notebook computer monitors, desktop computer monitors, and the like. Meanwhile, with an increased screen area of the liquid crystal display panel, a wide viewing angle, a high contrast, suppressing a change in color according to a viewing angle, and uniform display on a screen become particularly important. For this reason, various modes of liquid crystal display panels using twisted nematic (TN) liquid crystal cells, super twisted nematic (STN) liquid crystal cells, dual domain TN liquid crystal cells, vertical alignment (VA) liquid crystal cells, and the like are being developed. All these liquid crystal cells have inherent optical anisotropy due to their inherent liquid crystal alignment. Thus, to compensate for retardation due to the optical anisotropy of various modes of liquid crystal cells, a compensation film is required.
The optical anisotropy is divided into an in-plane retardation value (Rin) and a retardation value in a thickness direction (Rth) given by equations 1 and 2, respectivelyRin=(nx−ny)×d  (1)Rth=(nz−ny)×d  (2)Where nx is a refractive index in a slow axis direction (x-direction) in-plane of film, ny is a refractive index in a y-direction which is perpendicular to the x-direction in-plane of film, nz is a refractive index in a film thickness direction (z-direction), and d is the thickness of the film.
When there is a large difference between Rin and Rth, and either one of the values is dose to zero, the film can be used as a compensation film having uniaxial optical anisotropy, i.e., as a uniaxial optical compensation film. When absolute values of the two components are greater than 0, the film can be used as a compensation film having biaxial-optical anisotropy, i.e., as a biaxial-optical compensation film. The uniaxial optical compensation film is divided into an A-plate that satisfies the requirement of nx≠ny≅nz and a C-plate that satisfies the requirement of nx≅ny≠nz. The biaxial-optical compensation film satisfies the requirement of nx≈ny≠nz. When the in-plane retardation value (Rin) and the retardation value in a thickness direction (Rth) are appropriately adjusted, one biaxial-optical compensation film can function as both the A-plate and the C-plate, which are uniaxial optical compensation films.
While the in-plane retardation value of the compensation film can be controlled through secondary film processing such as precision stretching, it is difficult to control the retardation value in a thickness direction through secondary film processing. That is, when inherent birefringence in a thickness direction is small, the refractive index in a thickness direction can be controlled through biaxial stretching. In this case, the retardation value to be obtained in a thickness direction is generally smaller than the value to be obtained using a material having inherent birefringence in a thickness direction. Thus, it is preferable to manufacture a compensation film using a transparent material having a molecular arrangement of polymers in the direction of the thickness of film and a molecular arrangement in a planar direction different from each other. In particular, when only compensation of retardation by liquid crystal cells is considered, an ideal compensation film should have an optical axis in a mirror image of an optical axis of a liquid crystal cell. Therefore, for a liquid crystal display device using a VA liquid crystal cell or a TN liquid crystal cell in which the refractive index in a thickness direction is greater than the refractive index in a planar direction, a negative C-plate having negative birefringence in a thickness direction is required.
Polymers useful for the compensation film include discotic liquid crystal (U.S. Pat. No. 5,583,679), polyimide having a planar phenyl group on its backbone (U.S. Pat. No. 5,344,916, a cellulose ester film containing a low molecular weight aromatic compound which may be called as an optical phase retardation agent in a thickness direction (WO 2000/55657), a polycarbonate film (Japanese Patent Laid-Open Publication No. Hei 10-111412), a ring opening polymerized cyclic olefin-based polymer (a compensation film commercially available as trade name “Arton”), etc.
A method of synthesizing a cyclic monomer such as norbornene includes ring opening metathesis polymerization (ROMP), ring opening metathesis polymerization followed by hydrogenation (HROMP), copolymerization with ethylene, and homogeneous polymerization. Referring to reaction scheme 1, it can be seen that although the same monomer is used, polymers having different structures are obtained according to the type of polymerization methods used. These polymers have different physical properties.
Since the polymer synthesized by the ROMP has one unsaturated bond per a repeating unit, it has very poor thermal stability and oxidative stability and is mainly used as a thermosetting resin. The thermosetting resin prepared in this way can form a circuit board through reaction injection molding (RIM) (U.S. Pat. No. 5,011,730). However, the ROMP polymer has a low glass transition temperature in addition to poor thermal and oxidative stability. The HROMP polymer obtained by hydrogenating (addition of hydrogen) the ROMP polymer has increased oxidative stability and generally has about 50° C. higher glass transition temperature than the ROMP polymer. However, the HROMP polymer still has a low glass transition temperature due to an ethylene group located between cyclic monomers (cyclopentane units). In addition, complicated synthesis steps, high production costs, and poor mechanical properties restrict commercial use of the HROMP polymer.
The copolymer of ethylene and norbornene also has a low glass transition temperature of about 140° C. or less (DE Patent No. 109,224).
A polynorbornene-based polymer (hereinafter referred to as “addition-type polynorbornene-based polymer) obtained by addition polymerization of norbornene using a homogeneous catalyst has a bulky ring structure every repeating unit of its backbone. Thus, it has a very high glass transition temperature of about 200° C. or greater, which is at least 50° C. higher than the glass transition temperature of the ROMP polymer or the HROMP polymer. Since it is an amorphous polymer, there is no optical loss due to light scattering unlike a crystalline polymer and no light absorption in the visible range due to a conjugated double bond. Thus, it is suitable for use as a compensation film. In addition, an addition-type polynorbornene-based polymer film has negative birefringence in a thickness direction (Korean Patent Laid-Open Publication No. 2004-0005593). Large retardation in a thickness direction is sufficiently obtained even by solvent casting or coating without biaxial stretching. Since a biaxial-optical film can be obtained by uniaxially stretching the obtained film having a retardation in a thickness direction along a direction parallel to the film plane, the production process of a biaxial-optical compensation film can be simplified. The range of retardation values in a thickness direction and in a planar direction that can be obtained by uniaxial stretching is also large. The addition-type polynorbornene-based polymer is not easily melt-processed due to a high glass transition temperature and should be processed using a solvent casting method. Film that is not completely dried during solvent casting has a significantly reduced glass transition temperature due to the effects of solvent, which enables it to be stretched even at low temperatures.
As described above, the solvent-containing film can be stretched even at a temperature lower than the glass transition temperature of polymer and a plasticizer-containing film can also be stretched even at a temperature lower than the glass transition temperature of polymer. Japanese Patent Laid-Open Publication No. hei 4-204503 discloses a method of stretching a film using 2 to 10 wt % of a solvent based on the weight of solids in a solvent casting process. Japanese Patent Laid-Open Publication No. hei 5-113506 discloses a method of stretching a film, in which a methylene chloride solution is solvent cast, and then a partially dried film is stretched in a film proceeding direction (machine-direction: MD) at 200° C. or less when the amount of the solvent is 3 to 10 wt % based on the weight of solids. Japanese Patent Laid-Open Publication No. 7-92322 discloses a method of stretching polysulfone using 10 wt % or less of a solvent based on the weight of solids. Japanese Patent Laid-Open Publication No. 8-211224 discloses a method of stretching polysulfone using a plasticizer and 2 wt % of a solvent in a solvent casting process.
However, the methods of stretching the film obtained by solvent-casting an addition-type polynorbornene polymer after partially drying as described above have the following problems. That is, use of a plasticizer results in poor durability of the obtained film. When a single solvent is used and the partially dried film is stretched at a temperature higher than the boiling temperature of the solvent, this results in a relatively large change in modulus of the film over time due to volatilization of solvent, which makes it difficult to obtain uniform retardation.
FIG. 1 is a schematic cross-sectional view of an example of a conventional vertical alignment liquid crystal display 10 when a uniaxial optical film is used as a compensation film (U.S. Pat. No. 6,141,075).
Referring to FIG. 1, an A-plate 3 and a polarizer 7 composed of polyvinylalcohol (PVA) are sequentially laminated on a first surface of a liquid crystal cell 1. On one surface or, preferably, both surfaces of the polarizer 7, transparent protective films 5 and 9 composed of triacetate cellulose (TAC) are laminated with an adhesive for protecting the polarizer 7. A negative C-plate 3′ and a polarizer 7′ composed of PVA are sequentially laminated on a second surface of the liquid crystal cell 1. On one surface or, preferably, both surfaces of the polarizer 7′, transparent protective films 5′ and 9′ composed of TAC are laminated with an adhesive for protecting the polarizer 7′. Meanwhile, in another VA liquid crystal display, both the A-plate 3 and the negative C-plate 3′ can be formed on one surface of the liquid crystal cell 1 and the position of the A-plate 3 and the position of the negative C-plate 3′ as illustrated in FIG. 1 can be exchanged with each other.
However, the conventional liquid crystal display 10 having such a structure includes many film layers such as the A-plate 3, the negative C-plate 3′, polarizers 7 and 7′, and transparent protective films 5 and 9 on the liquid crystal cell 1, making it difficult to obtain a thin liquid crystal display and a simplified manufacturing process. Furthermore, the TAC protective film results in problems such as light leakage and reduction in the degree of polarization under high temperature and high humidity due to its relatively high moisture absorption property and has poor durability.
Japanese Patent Laid-Open Publication No. hei 10-111412 discloses a method of manufacturing a compensation film by uniaxially stretching a polycarbonate film obtained using a solution casting method to provide it with birefringence. Using this method, a retardation film with no bubbling and peeling off when being used, which has insignificant non-uniformity of retardation over the whole film and is suitable for color compensation, can be obtained. However, in the case of polycarbonate, it is difficult to realize a sufficient retardation value in a thickness direction by uniaxial stretching.
Meanwhile, stretching of organic polymers is generally carried out at the glass transition temperature of polymers or greater. For example, in Japanese Patent Laid-Open Publication No. hei 10-111412, a polycarbonate film is uniaxially stretched in a machine-direction at a temperature of 145 to 155° C., which is close to the glass transition temperature (about 150° C.) of the polymer. Japanese Patent Laid-Open Publication No. 2001-215332 discloses a method of manufacturing a ring opening polymerized cyclic polynorbornene-based film. In this method, the ring opening polymerized cyclic polynorbornene-based film (e.g. film commercially available as a trade name “Arton”) is sufficiently dried and stretched at a temperature of at least 30° C. greater than, in particular at a temperature of 32 to 60° C. greater than, the low glass transition temperature (<200° C.).
However, a film from an addition-type polynorbornene-based polymer (hereinafter, referred to as an “addition-type polynorbornene-based film) more preferable as a compensation film has a high glass transition temperature of 200° C. or greater. Thus, when the stretching temperature is set to be higher than the glass transition temperature of the polymer as in general cases, stretching is carried out at too high a temperature, resulting in yellowing in the addition-type polynorbornene-based film due to pyrolysis. A conjugated double bond created in the molecular structure of film by pyrolysis absorbs light in the range of a short wavelength of visible light, and thus can reduce the light transmittance of film. In addition, the film is brittle, and thus is easily broken. Thus, when the sufficiently dried addition-type polynorbornene-based film is stretched at a temperature higher than the glass transition temperature to manufacture a biaxial-optical film having in-plane retardation, various problems can occur.
Meanwhile, a method of manufacturing a biaxial-optical polynorbornene-based film has been proposed, including obtaining a polynorbornene-based polymer solution using a highly volatile low-boiling solvent such as methylene chloride, casting the polynorbornene-based polymer solution on a substrate, partially drying the solution to obtain a partially dried film, peeling off the partially dried film from the substrate, and stretching the resulting film. The cast method using only the highly volatile low-boiling solvent is advantageous in view of high productivity and low production costs, but has disadvantages that it is difficult to constantly control the amount of a residual solvent in the partially dried film during stretching and uniformly control an in-plane retardation value of the obtained polynorbornene-based film due to relatively fast volatilization of solvent.